1. Field of the Invention
The present invention relates to a light guide plate for guiding light, a surface light source device having the light guide plate, and a display device having the surface light source device.
2. Description of Related Art
Surface light source devices, which emit light from a planar surface, are widely used e.g. as a backlight which is incorporated in a liquid crystal display device and illuminates a liquid crystal display panel from the back (see e.g. JP 2007-227405A). Surface light source devices are roughly classified into direct-light type devices which have a light source disposed right behind an optical member, and edge-light type (also called side-light type) devices which have a light source disposed lateral to an optical member.
Edge-light type surface light source devices have a structural feature that enables design of thin surface light sources as compared to direct-light type surface light source devices. Because of the structural feature, edge-light type surface light source devices have heretofore been applied principally in display devices for laptop personal computers (PCs).
In an edge-light type surface light source device, a light guide plate is provided lateral to a light source, and light from the light source enters the light guide plate through the side surface (light entrance surface) of the light guide plate. The light that has entered the light guide plate repeats reflection at a pair of opposing main surfaces and travels in the light guide plate in a direction (light guide direction) almost perpendicular to the light entrance surface. The light traveling in the light guide plate is subject to the optical action of the light guide plate and is caused to gradually exit the light exit surface as it travels in the light guide plate. The amount of light, exiting the light exit surface of the light guide plate, can thus be prevented from varying greatly along the light guide direction. In an exemplary light guide plate, a light scattering component is dispersed in the light guide plate in order to change the direction of light traveling in the light guide plate, thereby allowing the light to exit the light guide plate gradually at various positions on the light guide plate along the light guide direction.
However, in current edge-light type surface light source devices, the amount of light exiting a light exit surface is not completely uniform along the light guide direction: the amount of the exiting light tends to be large locally in an area of the light exit surface, lying close to a light source. A viewer, viewing the display surface of a display device incorporating a surface light source device, can well perceive an increase in the brightness of an image as displayed in a central area of the display surface, corresponding to a central area of the light exit surface of the surface light source device, but cannot well perceive an increase in the brightness of an image as displayed in a peripheral area of the display surface, corresponding to a peripheral area of the light exit surface of the surface light source device.
Thus, the fact that the amount of exiting light is large locally in an area of a light exit surface, lying close to a light source, is undesirable not only from the viewpoint of ensuring the uniformity of the amount of exiting light along the light guide direction but also from the viewpoint of efficiently using a limited amount of light emitted by a light source. It would therefore be highly desirable if the amount of light exiting an area of a light exit surface, lying close to a light source, can be reduced while the amount of light exiting a central area of the light exit surface, lying remote from the light source, can be increased.
The present invention has been made in view of the above situation. It is therefore an object of the present invention to provide a display device, a surface light source device and a light guide plate which can effectively increase the brightness perceived by a viewer.
Nowadays, with the development of light sources that emit light with high linearity, as typified by a light emitting diode, edge-light type surface light source devices have been put into use also in display devices having a larger size than laptop PCs, for example home televisions. However, it has turned out that when a linear array of light emitting diodes (LEDs) is used as a light source, in-plane variation of brightness (more precisely luminance) is produced in an area of a display surface (light emitting surface), lying in the vicinity of a light entrance surface that faces the light source. More specifically, in the area of the display surface (light emitting surface), lying in the vicinity of the light entrance surface, a pattern of alternating bright and dark portions is formed along the LED arrangement direction at the same pitch as the pitch of the array of LEDs.
It has also turned out that the in-plane variation of brightness is marked when a light guide plate is used which has linear prisms extending in the light guide direction and provided on the light exit side. In particular, when a light guide plate with prisms is used, in-plane variation of brightness is observed in a larger area of a display surface (light emitting surface).
It is expected that as the emission intensity of LED increases in the future, a decreased number of LEDs will be arranged at a larger spacing in a light source in order to reduce the cost of a display device. It is, therefore, possible that the above-described in-plane variation of brightness may be observed more clearly in a wider area of a display surface (light emitting surface).
It would be highly desirable if the use of a light guide plate according to the present invention can obscure the in-plane variation of brightness in an area of a display surface (light emitting surface), lying in the vicinity of a light entrance surface, as observed when a light source is used which is comprised of a plurality of point-like light emitters arranged in a line.